Method for making an article having a gradient of composition

ABSTRACT

The present invention relates to a method of making a rectangular bar have a gradient composition. In optical or ophthalmic applications, the gradient of composition ensures formation of a corresponding gradient of optical property, such as refractive index or optical density. The glass material is manufactured using a device containing several reservoirs, all designed to contain glass in the fluid state. According to the invention, the composition of the glass varies from one reservoir to the other so that the refractive indices, for example, of the three compositions are different, with an incremental progression. The fluid glass is deposited one composition on top of the next and then heated, creating a certain degree of interdiffusion between the layers. The glass is then heat-treated to create the desired property profile.

Provisional Application No. 60/009,854 filed Jan. 16, 1996.

The present invention relates to a method and a device for themanufacture of a bar with rectangular cross section made of a materialpresenting a gradient of composition perpendicular to two oppositelongitudinal sides of the bar and, more particularly, to such a methodand a device which allow the manufacture of a glass bar having onecharacteristic, such as refractive index, optical density, ionicconductivity, etc., which presents a predetermined profile gradient,established as a result of a variation in its composition, perpendicularto said opposite sides of the bar.

Optical components are used today, such as lenses made of glass whichpresents a refractive index gradient which is parallel to the axis ofthe lens. To make such optical components, the starting material is aglass block whose composition varies along the axis of the component tobe prepared, which variation is established using various techniques,such as ion exchange (see French Patent Application No. 2,504,515),impregnation with the so-called "sol gel" technique (see U.S. Pat. No.5,069,700), the use of starting glass compositions made of powder (U.S.Pat. No. 4,883,522), the use of selectively doped porous glass (U.S.Pat. No. 4,620,861), the assembly by melting of platelets made of glasswith varying composition (U.S. Pat. No. 4,929,065), or other techniquessuch as exposure to neutron radiation, deposition of glass in the vaporphase, etc, etc.

The techniques used in the past share the feature that they applydiscontinuous manufacturing processes, that is, in lots; this continuityimplies downtimes which have a negative effect on productivity and thusalso on the lowering of production costs. The gradients of indicesobtained also were found to be insufficient in some applications and theprofile insufficiently controlled.

The present invention therefore has the purpose of implementing a methodand providing a device which allows the manufacture of bars withrectangular cross section made of a material which presents a gradientof composition perpendicular to two opposite longitudinal sides of thebar, and notably bars made of glass for the preparation of opticalcomponents with an axial gradient of composition, which bars presentnone of the above-mentioned drawbacks of the known methods and devices.

This purpose of the invention is achieved, as well as others which willbecome apparent upon reading the following description, with a methodfor the manufacture of a bar with rectangular cross section made of amaterial presenting a gradient of composition perpendicular to twoopposite longitudinal sides of the bar, which material is in the fluidphase at a temperature above ambient temperature, and which method ischaracterized in that a) at least the first and second compositions ofthis material are prepared in the fluid phase, b) a sheet of the firstcomposition is poured continuously on a flat support, c) a sheet of thesecond composition is poured continuously on the sheet of the firstcomposition, d) the interdiffusion of the materials of the two sheets bya thermal method is ensured so as to obtain the desired gradient ofcomposition in the cooled bar.

With the method of the invention, a bar can be prepared in a continuousprocess, which bar consists of a material such as glass which presentsthe required transverse gradient of composition, where the continuity ofthe method allows for of high productivity and reduced manufacturingcosts.

For the implementation of this method, the invention provides a devicewhich comprises a support which advances on a looped travel path whichpresents a flat part, where a number of reservoirs each receive one ofthe compositions of an equal number of different compositions of thematerial each reservoir being equipped with means for the formation of asheet of the material which flows by gravity from the reservoir, thereservoirs being spaced at intervals along the flat part of the travelpath of the support so that said sheets flow out one above the other,above the support.

According to one embodiment of the device according to the invention,the device also comprises means for the transverse cutting of the barformed into bar elements, heating means and means to load these elementsin these heating means so as to ensure the development of the process ofinterdiffusion of the sheets of each element until the desired gradientof composition is established in said element.

Other characteristics and advantages of the present invention willbecome apparent upon reading the following description and examining thedrawing in the appendix in which:

FIG. 1 is a diagrammatic representation of the structure of a glass barobtained by the method according to the present invention,

FIG. 2 is a diagram of a device designed for the implementation of themethod according to the invention,

FIG. 3 is a diagram of a variant of a part of the device of FIG. 2, and

FIG. 4 is a diagram illustrating the structure and the operation of themeans for loading glass bar elements in heating means of the device ofFIG. 2.

The present invention relates to the manufacture of a bar 1 (see FIG. 1)made of a vitreous material, in particular an optical or ophthalmicglass, presenting a rectangular cross section and a gradient ofcomposition along arrow F, that is, along a direction perpendicular tothe two opposite sides of said bar.

In the optical or ophthalmic applications considered primarily, but notexclusively, by this invention, the gradient of composition ensures theformation of a corresponding gradient of an optical property such asrefractive index or optical density. By cutting the bar 1 transversely(either perpendicular to its axis, parallel to the arrow F₁) preparedaccording to the invention, into separate elements, preforms can bemade, for example, of optical lenses which are then shaped by grindingand mechanical polishing, or by molding, and which present an opticalaxis which is parallel to the arrow F. These lenses then present, oncethey are finished, an axial gradient of the desired optical property,for example, the refractive index.

To prepare such lenses, the glass bar of FIG. 1 is first manufacturedusing the device represented in FIG. 2. The latter comprises nreservoirs 2₁, 2₂, 2₃, (n=3 in the example represented), all designed tocontain glass in the fluid state. Classically, the glass is maintainedin this state by heating means (not shown) which are incorporated in thereservoirs.

According to the invention, the composition of the glass varies from onereservoir to the other so that the refractive indices, for example, ofthe three compositions, are different, with an incremental progression,for example.

The bottom of each reservoir 2₁, 2₂, 2₃ is equipped with a casting slit3₁, 3₂, 3₃, respectively, designed to form a sheet of glass in the fluidstate from the glass contained in the reservoir and to deposit thissheet onto a support 4, consisting, for example, of a belt which travelsin a closed-loop path around two rollers 5₁ and 5₂ which define betweenthem a flat travel path for the belt 4.

The reservoirs 2₁, 2₂ and 2₃ and the slits 3₁, 3₂ and 3₃ are spaced atintervals along this travel path so that the sheet of glass 6₁ exitingfrom slit 3₁ falls directly onto belt 4, possibly assisted by a shoe 7placed at the drop point, so that sheet 6₂ exiting from slit 3₂ fallsonto sheet 6₁ placed on the belt, and sheet 6₃ exiting from reservoir 3₃falls onto sheet 6₂. Heating means 8₁, 8₂ and 8₃ are arranged inparallel to belt 4 to maintain the sheets in paste-like condition (at atemperature much higher than the vitreous transition temperature T_(g))as they are stacked on the belt. Walls (not shown), made of graphite,for example, limit the transverse extension of the sheets on the belt.

At the moment of the forming of the bar, the interface between the twosheets can be slightly arched because of the friction on the graphitewalls and the effect of surface tension. This defect can be corrected,if necessary, using rollers, such as the lamination roller 10 shown inFIG. 3, placed downstream from each one of the slits, in contact withthe sheet of material deposited through the slit.

One can see that in this manner that a composite glass bar with arectangular cross section is formed, which leaves the belt 4 (to theright of the point of view represented in FIG. 2), the composition ofthis bar varying abruptly at the two interfaces between the sheets 6₁and 6₂ on the one hand, and 6₂ and 6₃ on the other.

Indeed, the phenomenon of heat diffusion which can cause a certaindegree of interdiffusion between the two sheets at each one of theseinterfaces during the stacking of the sheets on the belt has very lowsensitivity because such a diffusion requires a high temperature and along contact time. The glass bar which leaves the belt thus does notpresent a composition which varies progressively in the direction of itsthickness, when it may in fact be desirable to establish such aprogression for certain optical components with a gradient of refractiveindices, for example.

As explained in French Patent Application No. 95,04646 filed on Apr. 19,1995, by the applicant, the required gradient of composition can beobtained then by the interdiffision at high temperature of chemicalelements which are capable of significantly modifying the value of therefractive index of glass, as a function of their local concentration inthe glass. The interdiffusion takes place at the interfaces of the glasssheets. The selection of the glasses for these sheets depends on thedesired difference in indices and the required type of profile. In theextreme case where there is no interdiffusion, a profile withdiscontinuous composition is obtained (stepwise). With reference to FIG.2 of the above-mentioned patent application, the appearance of "S"profiles of indices can be examined, which can be obtained byinterdiffusion, the inflection point of the profile being located in thevicinity of an interface between glass sheets or layers.

To establish a gradient of indices which varies progressively over theentire thickness of glass bar 1 by means of the device according to theinvention, the glass bar must undergo, at the exit point of belt 4, aheat treatment at high temperature (temperature of diffusion) forseveral tens of hours, for example.

According to a particular embodiment of the present invention, thistreatment can be achieved by first cutting bar 1 into elements 1₁, 1₂, .. . 1_(i) . . . at a cutting station 11. This cutting operation isregular and it is conducted in the standard manner, with triggering bythermal shock or heat or sawing, as is well known. The length of theelements 1_(i) is selected, for example, as a function of the dimensionsof optical lenses with a gradient of indices to be formed by grinding orpolishing, or by molding, of the material for each element, which thenconstitutes a preform for the desired lens.

Preform 1_(i), so individualized, is placed by automatic mechanicalmeans (not shown) in the mold 12 which then is conveyed into heatingmeans 13 where the interdiffusion of the portions of sheets 6₁, 6₂ and6₃ which constitute the preform 1_(i) will take place. These meansconsist of an arch through which a moving belt 14 passes, which conveysthe preform 1_(i), over the entire length of the arch, between heatingelements 15₁ and 15₂.

The latter establish in the arch the temperature profile T shown in FIG.2. This profile comprises a part a in which the temperature of thepreform gradually increases up to the temperature T_(dif) of diffusion,a part b which corresponds to the greatest part of the length of thearch, where the diffusion proper takes place, and a part c where thetemperature of the preform 1_(i) is lowered to a value which allows theremoval from the molds of the preforms at the exit of the arch, so thatsubsequent process can be performed on these preforms to make lensesfrom them with an axial gradient of indices, the axis of the lens beingthen oriented in parallel to the thickness of the preform (vertical,from the point of view of FIG. 2).

The parameters which allow the modification of the profile of indicesobtained are the diffusion time (corresponding to the time it takes thepreform to travel through the part of arch 13 which corresponds to thethreshold b of the temperature profile T) and the positions of theinterfaces of the sheets. By increasing the number n of the sheets, itis possible to shape the form of the profile more precisely and toobtain, for example, a parabolic profile. Regardless of the number ofsheets, the densities of the glasses which comprise them must beselected so that they are stacked one atop the other in decreasing orderof density so as to preserve the gradient during the passage in thearch, where the glass can return to the liquid phase because of the hightemperature established in this arch. In the latter case, the laminationrollers described in connection with FIG. 3 are no longer necessary.

The positioning of the interfaces also depends on the thickness of thesheets. In this regard, the parameters which can be modified are theflow rates of the glass from reservoirs 2_(i) (and the ratios of theseflow rates) as well as the speed of the belt 4, which can ensure apredetermined drawing of the sheets and thus a modification of theirthicknesses. As an illustrative and nonlimiting example, the method andthe device according to the invention allow the manufacture of 100 kg/hof preforms into a bar with a rectangular cross section 80×10 mm, formedof two glass sheets with a thickness of 5 mm (made of a commercial whiteglass or a lead-containing glass, for example).

For this purpose, an arch 13 with a length of 30 m is constructed, whichis heated at a diffusion temperature of 1200° C. The mold which supportsthe preform is made of graphite or of boron nitride. An inert atmosphere(nitrogen, for example) is then required in the arch to ensure themaintenance of the temperature of the mold.

The glass bar exits the belt 4 at a speed of 1.157 cm/sec. The speed ofthe mold in the preform is 1 m/h for a residence time of 30 h. Since thespeed (30 m/h) of belt 14 in the arch is slow compared to the exit speedof the glass bar from belt 4, it is advantageous to have loading means16 in the arch of the preforms cut into the bar, which absorb thedifferences in the glass flows.

According to the present invention, these loading means 16 have the formshown in FIG. 4, which represents diagrammatically in a top view glassbar 1 exiting belt 4, to be cut at station 11. After the cutting,mechanical transfer means (not shown) place each preform in a mold, andthe molds so loaded are aligned transversely to belt 14 of arch 13before being placed on this belt. Thus, several preforms are heattreated in the arch in parallel. It is sufficient to select the width ofthe belt 14 which exactly adapts the flow rate of belt 4 to that of belt14. With the numbers indicated above, this adaptation is obtained byplacing preforms in molds side by side in arch 41 and in that case, belt14 has an approximate width of 3.3 m.

It now becomes apparent that the invention does in fact realize thestated purpose, that is to provide a method and a device for themanufacture of elements with rectangular cross section made of amaterial presenting a gradient of composition perpendicular to twoopposite sides of the element, which do not present the drawbacks of thediscontinuous methods of the prior art. Thanks to its continuouscharacter, the method according to the invention allows the increasingof productivity and the decreasing of production costs. It should alsobe noted that the method according to the invention allows themodification, by very simple means, of the composition of the bars bythe addition of additional sheets, without changing the subsequent stepsof heat treatment.

Naturally, the invention is not limited to the embodiment which has beendescribed and represented only as an example. Thus, the invention is notlimited to the manufacture of lenses with a gradient of refractiveindices for optical or ophthalmic use; it can also be applied to themanufacture of filters with variable optical density or blocks ofmaterials with variable ionic conductivity, for example.

What is claimed is:
 1. A method of producing a bar of vitreous materialhaving a rectangular cross-section, having two opposed, longitudinalsides, and having a compositional gradient perpendicular to those sides,the method being characterized by,a. providing at least a first glasshaving a given composition and a second glass having a differentcomposition, both glasses being provided in a fluid state, b.continuously pouring the first glass onto a flat support to form a sheetof glass on the support, c. continuously pouring the second glass ontothe surface of the first glass sheet to form a second sheet of glasshaving an interface with the first glass sheet, d. thermallyinterdiffusing the two glass compositions across the interface toproduce the desired compositional gradient in the bar, and e. coolingthe thermally interdiffused bar thus produced to a solid, wherein apredetermined profile of the gradient of composition is prepared byselecting the position of the interface of the sheets and by regulatingthe temperature and the duration of their thermal interdiffusion. 2.Method according to claim 1, characterized by regulating the ratio ofthe pouring flow rates of the two sheets to adjust the position of theinterface.
 3. Method according to claim 1 characterized by preparingmore than two different glass compositions in the fluid state, theglasses having different densities, and successively pouring a sheet ofeach of these glass compositions one atop the other on the flat supportin order of decreasing density.
 4. Method according to claim 3characterized by transversely cutting the solid bar into bar elementsand thermally treating the bar elements to effect a desired degree ofinterdiffusion of the layers which comprise them.
 5. Method according toclaim 3 characterized by thermally treating the bar elements toestablish a gradient of composition that corresponds to a predeterminedgradient of refractive indices.
 6. Method according to claim 1characterized by continuously cutting the solid bar into bar elementsand thermally treating the bar elements to ensure interdiffusion of thelayers in the elements.
 7. Method according to claim 6 characterized bythermally treating the bar elements to establish a gradient ofcomposition that corresponds to a predetermined gradient of refractiveindices.
 8. Method according to claim 1 which comprises providing a flatsupport, arranging at least two reservoirs of molten glass in a spacedrelationship above the flat support, continuously delivering a sheet ofglass from each reservoir by gravity flow, and successively depositingthe sheets atop one another on the support to form a stack.